In conventional window blinds, multiple slats are connected trapezoidal ropes that are extended between a top rail and a bottom rail, one or more pull ropes run through the slats and extend from the bottom rail to the top rail. The bottom rail will be raised towards the top rail under an upward force applied to the pull rope, in such a way, the slats will be retracted from bottom to top. Due to restrictions of the blinds system and driving manner, the pull rope is longer than the total length of the window blinds after the window blinds are retracted, which brings potential strangulation hazard to children.
Cord-free window blinds are developed to solve the above problems, which use a spring driver as a driving source connected to a winding reel or a winding roller, and a pull rope wound around the winding reel or winding roller. On operation, the spring driver supplies a lift force to counteract the resultant force of the gravity of the slats and the force applied by a hand, by the means of balance force of the friction, the gravity, the force provided by the spring driver and the force applied the hand, the window blind can be retracted or extended at a desired position. However, such a balance is difficult to achieve by the spring driver. When the window blind is pulled, the force supported by the spring driver is reduced as the slats are support by the trapezoidal ropes. Contrarily, when the window blind is withdrawn, the force supported by the spring driver is increased due to the loads of the bottom rail and the slats. Thus some problems will be generated if the lift force provided by the spring driver is constant. For example, since the force supported by the spring driver is reduced gradually when the window blind is pulled down, thus the pull force of the hand must be increased gradually and will reach to the maximum at the bottom, and once the hand is loosed, the window blind will be rebounded due to the inertia. For preventing such a rebound, a counterweight must be added at the bottom of the window blind such as the bottom rail. When a hand lifts the bottom rail to retract the blind, the force supported by the spring driver is increased gradually, the lift force of the hand must be increased accordingly and will reach to the maximum at the top. Under this situation, the window blind may not be maintained at the top position, but fall down slowly. For improving this problem, one or more driving element is added at the top rail to increase the friction to balance the force supported by the spring driver.
However, the above structures are complicated and the cost is increased. In the mass production, the frictions become indefinite due to assembling process and different using ways, thus the rebound force, the frictions and the gravity may still mismatch to bring the unbalanced force.
Thus, there is a need to provide an improved driving assembly and a window blind to overcome the drawbacks mentioned above.